1. Field of the Invention
The present invention relates to a method for reducing metal ions with nanosilicate platelets (NSP), particularly for producing an aqueous solution of stably dispersed reduced metal nanoparticles. This method and the metal nanoparticles are suitable as nano-composite materials and biomedical materials, for example, electrical devices, catalysts, conductors, biosensor, pharmaceutical and polymers. The present invention also relates to the product obtained by the above method, namely stably dispersed metal nanoparticles including Ag nanoparticles and Cu nanoparticles.
2. Related Prior Arts    1. In recent years, the inventor(s) of the present invention have developed many processes and products for modifying clay and nanosilicate platelets (NSP), for example, those disclosed in U.S. Pat. Nos. 7,125,916, 7,094,815, and 7,022,299 or Publication Nos.: US 2006-0287413-A1 and US 2006-0063876A1. In addition, a series of polymeric surfactants used for modifying smectic clay are obtained. By adding the surfactants, the clay can be directly exfoliated through ionic exchanging reactions, then the exfoliated clay is extracted with NaOH water/toluene biphase solution to obtain pure nanosilicate platelets. The nanosilicate platelets have a high aspect ratio (on average, 100×100×1 nm3), high specific surface area (700˜800 m2/g) and strong charges (ca. 20,000 ions/platelet), and generally there are about 4×1016 platelets per gram thereof. FIG. 1 shows the basic structure of the silicate platelets. With the above characteristics, surface charges of the nanosilicate platelets vary with pH, that is, negative when above the isoelectric point (IEP, pH 6.4) and positive when below the IEP. Aggregation occurs when the surface charges are positive.
Due to the high charge density of ions, the nanosilicate platelets in a water solution can strongly adsorb metal ions and thus precipitate. For example, a black precipitate will be immediately generated after adding AgNO3 into a water solution of the nanosilicate platelets. Such reaction is vigorous and will result in quick aggregation.
So far, methods for producing silver nanoparticles are classified into physical methods and chemical methods. The physical method usually demands expensive equipment for highly-vacuum vaporization or e-beam. The chemical method uses reducers to reduce the silver ions as atoms and then a stabilizer is used to control the size of the particles. The general reducers include NaBH4, formaldehyde, alcohol, hydrazine (H2N—NH2); and the stabilizers include sodium citrate, glucose, sodium dodecyl sulfate, polyvinyl pyrrolidone (PVP), dendrimer, etc.
In the present invention, the nanosilicate platelets are provided for the purpose of reducing metal ion with a proper stabilizer to obtain nanoparticles.